HK1191768A - Secondary synchronization signal detection with interference cancelation for lte - Google Patents

Abstract

The present invention is directed to secondary synchronization signal detection with interference cancelation for LTE, wherein methods and systems for removing interference from strongly powered SSS sequences in a received signal so that comparatively weakly powered SSS sequences in the received signal can be detected are disclosed. The methods and systems can perform a first cell search using the received signal to detect a strongly powered SSS sequence. Using a known SSS sequence corresponding to the strongly powered SSS sequence, the channel over which the strongly powered SSS sequence is received can be estimated. The estimated channel can then be used to determine the contribution of the strongly powered SSS and PSS sequence to the received signal so that it can be canceled. With the contribution of the strongly powered SSS and PSS sequence canceled from the received signal, a second cell search can be performed using the received signal to detect a weakly powered SSS sequence.

Description

Secondary synchronization signal detection with interference cancellation for LTE

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional patent application No. 61/562,196 filed on day 21, 2011 and from U.S. provisional patent application No. 61/674,567 filed on day 23, 7, 2012, the entire contents of which are incorporated herein by reference.

Technical Field

The present application relates generally to cellular networks and more particularly to synchronization signal detection in cellular networks.

Background

Fig. 1 shows a Long Term Evolution (LTE) cellular network 100 distributed in a terrestrial region 110, referred to as a cell, each cell being served by a base station 120. The cells 100 are geographically connected together so that LTE terminals 130 (e.g., mobile phones, laptops, tablets, etc.) can wirelessly communicate with a core network (not shown) in a wide area via the base station 120.

Before an LET terminal can communicate over an LTE cellular network, such as the LET cellular network 100 in fig. 1, the LTE terminal typically needs to perform a cell search to obtain frequency and symbol synchronization with a cell and detect a physical layer identity of the cell. For example, an LTE terminal may perform a cell search to obtain synchronization with a cell and detect the physical layer identity of the cell or some other cell in which it is located. In addition, the LTE terminal may continue with cell search to obtain synchronization with other nearby cells and detect physical layer identities of other nearby cells. This allows an LTE terminal to move from one cell to another while maintaining sufficient connectivity with the LTE cellular network. For example, if the signal quality supported by the current cell is lower than the signal quality supported by one of the other nearby cells due to movement of the LTE terminal, communication with the current cell may be handed over to the nearby cell supporting a higher signal quality.

Two synchronization signals, a Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS), are broadcast from base stations in an LTE cellular network to assist cell search. The time domain position of these two signals within an LTE frame is typically constant from frame to support synchronization and depends on whether the LTE cellular network is operating in frequency division multiplexing mode (FDD) or time division multiplexing mode (TDD). As shown in fig. 2, a general LTE frame structure 200 lasts 10ms and includes two 5ms half-frames. Each field is further divided into 5 sub-frames (0 to 4 and 5 to 9) each lasting 1 ms. A subframe typically carries 14 Orthogonal Frequency Division Multiplexing (OFDM) symbols. In LTE cellular networks operating in FDD mode, PSS is typically transmitted in the last OFDM symbol of subframes 0 and 5, while SSS is typically transmitted in the second to last OFDM symbol in the same subframe, just before PSS. In an LTE cellular network operating in TDD mode, PSS is typically transmitted in the third OFDM symbol of subframes 1 and 6, while SSS is typically transmitted in the last OFDM symbol of subframes 0 and 5.

During cell search, LTE terminals use PSS and SSS to obtain frequency and symbol synchronization with a cell and detect the physical layer cell identity of the cell. Detecting the physical layer cell identity involves obtaining a group identity NID1= (0,..,. 167) from the SSS sequence broadcast by the base station, and an identity NID2= (0, 1, 2) within the group identified by NID1 from the PSS sequence broadcast by the base station. Group identification NID1 is detected from the SSS sequence after the identification NID2 within the group is detected from the PSS sequence. After detecting NID1 and NID2, the relationship of NID = (3 × NID1) + NID2 may be used to determine the physical layer unit identification, where NID is the physical layer unit identification. Because there are 168 unique group identifications NID1 and three unique identifications NID2 within each group, there are a total of 504 unique physical layer identifications in the LTE cellular network.

Typically, an LTE terminal receives strong power synchronization signals (i.e., PSS and SSS) from certain base stations and weaker power synchronization signals from other base stations. At LTE terminals, the strong power synchronization signal may drown out the weak power synchronization signal, which prevents the LTE terminal from obtaining synchronization with the cells that generated the weak power synchronization signal and from detecting the physical layer identities of these cells. Obtaining synchronization with these cells and detecting the physical layer identities of these cells may be advantageous for several reasons, including, for example, handing over communications from one cell to another due to movement of LTE terminals.

Disclosure of Invention

Disclosed herein is a method for detecting a first Secondary Synchronization Signal (SSS) sequence and a second secondary synchronization signal sequence in a received signal, the method comprising: detecting a first secondary synchronization signal sequence in a received signal; estimating a channel receiving the first secondary synchronization signal sequence; estimating a contribution of the first secondary synchronization signal sequence to the received signal using the channel estimate; cancelling the contribution estimate of the first secondary synchronization signal sequence from the received signal to produce an interference cancelled received signal; and detecting a second secondary synchronization signal sequence using the interference-cancelled received signal.

Preferably, the method further comprises: the contribution estimate of the third secondary synchronization signal sequence is cancelled from the received signal prior to detecting the second secondary synchronization signal sequence.

Preferably, estimating the channel further comprises: a received signal and a known secondary synchronization signal sequence corresponding to the first secondary synchronization signal sequence are used.

Preferably, estimating the channel further comprises: a frequency domain representation of the samples of the received signal is multiplied by a known secondary synchronization signal sequence corresponding to the first secondary synchronization signal sequence.

Preferably, the method further comprises: samples of the received signal are obtained based on a position of the first secondary synchronization signal sequence in the received signal.

Preferably, the method further comprises: noise is removed from the sampled frequency domain representation based on an estimated energy of an element (element) in the frequency domain representation and an estimated delay spread of the channel.

Preferably, the method further comprises: acquiring synchronization with the cell using the second secondary synchronization signal sequence.

Preferably, the method further comprises: a physical layer cell identity of the cell is determined using the second secondary synchronization signal sequence.

Also disclosed herein is a terminal for detecting a first Secondary Synchronization Signal (SSS) and a second secondary synchronization signal sequence in a received signal, the terminal comprising: a channel estimator configured to estimate a channel on which the first secondary synchronization signal sequence is received; an interference canceller configured to generate an interference-cancelled received signal by: estimating a contribution of the first secondary synchronization signal sequence to the received signal and cancelling a contribution estimate of the first secondary synchronization signal sequence from the received signal using the channel estimate, and estimating a contribution of the first primary synchronization signal sequence to the received signal and cancelling a contribution estimate of the first primary synchronization signal sequence from the received signal using the channel estimate; and a detector configured to detect the second secondary synchronization signal sequence using the interference-cancelled received signal.

Preferably, the sequence detector is further configured to detect the first secondary synchronization signal sequence in the received signal.

Preferably, the channel estimator is further configured to use the received signal and a known secondary synchronization signal sequence corresponding to the first secondary synchronization signal sequence.

Preferably, the channel estimator is further configured to multiply the frequency domain representation of the samples of the received signal by a known secondary synchronization signal sequence corresponding to the first secondary synchronization signal sequence to estimate the channel.

Preferably, the samples of the received signal are obtained based on the position of the first secondary synchronization signal sequence in the received signal.

Preferably, the channel estimator is further configured to remove noise in the sampled frequency domain representation based on the estimated energy of the elements in the frequency domain representation and the estimated delay spread of the channel.

Preferably, the sequence detector is further configured to use the second secondary synchronization signal sequence to obtain synchronization with the cell.

Preferably, the sequence detector is further configured to determine a physical layer cell identity of the cell using the second secondary synchronization signal sequence.

Also disclosed herein is a terminal for detecting a first sequence and a second sequence in a received signal, the terminal comprising: a channel estimator configured to estimate a channel receiving the first sequence; an interference canceller configured to estimate a contribution of the first sequence to the received signal using the channel estimate and configured to cancel the contribution estimate from the received signal to produce an interference cancelled received signal; and a detector configured to detect the second sequence using the interference-cancelled received signal.

Preferably, the channel estimator is further configured to use the received signal and a known sequence corresponding to the first sequence.

Preferably, the channel estimator is further configured to multiply the frequency domain representation of the samples of the received signal by a known sequence corresponding to the first sequence to estimate the channel.

Preferably, the channel estimator is further configured to remove noise in the sampled frequency domain representation based on the estimated energy of the elements in the frequency domain representation and the estimated delay spread of the channel.

Drawings

The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate embodiments of the present disclosure and, together with the description, further serve to explain the principles of the embodiments and to enable a person skilled in the pertinent art to make and use the embodiments.

Fig. 1 illustrates an LTE cellular network in accordance with embodiments of the present disclosure.

Fig. 2 illustrates a general LTE frame structure according to an embodiment of the present disclosure.

Fig. 3 shows a flow diagram of a method of detecting a weak-power SSS sequence in an LTE cellular network using interference cancellation (interference cancellation), according to an embodiment of the present disclosure.

Fig. 4 illustrates an exemplary LTE terminal according to an embodiment of the present disclosure.

Embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.

Detailed Description

In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the invention. It will be apparent, however, to one skilled in the art that embodiments, including structures, systems, and methods, may be practiced without these specific details. The descriptions and representations herein are the means used by those skilled in the art, or the means used by others skilled in the art, to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.

References in the specification to "one embodiment," "an example embodiment," etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but individual embodiments may not necessarily include the particular feature, structure, or characteristic. Moreover, these terms do not necessarily refer to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

Described below are methods and systems for removing interference from one or more strong power SSS sequences in a received signal such that one or more weak power SSS sequences in the received signal can be detected. The method and system enable a first cell search using a received signal to detect a high power SSS sequence. Using the known SSS sequence corresponding to this high power SSS sequence, the channel receiving the high power SSS sequence can be estimated. The estimated channel may then be used to determine an estimate of the contribution of the strong power SSS sequence to the received signal so that it can be cancelled. The estimated channel can further be used to determine an estimate of the contribution of the PSS sequence of the cell transmitting the strong power SSS sequence such that the contribution of the PSS sequence can also be cancelled from the received signal. With the estimated contribution of the strong power SSS sequence and the PSS sequence cancelled from the received signal, a second cell search can be conducted using the received signal to detect the weak power SSS sequence.

Once detected, the low power SSS sequence may be used to obtain synchronization with the cell from which the low power SSS sequence originates and detect the physical layer identity of the cell. Obtaining synchronization with this cell and detecting the physical layer identity of this cell is beneficial for several reasons, including handover of communications from the current cell to the cell corresponding to the weak power SSS sequence due to, for example, movement of LTE terminals.

It should be noted that descriptions of SSS/PSS sequences as "strong power" refer to those SSS/PSS that can be detected from received signals during cell search without cancelling estimated contributions to the received signals from one or more other SSS sequences, while descriptions of SSS sequences as "weak power" refer to those SSS/PSS that cannot be detected from received signals during cell search without cancelling estimated contributions to the received signals from one or more other SSS sequences.

It should also be noted that although the methods and systems of the present disclosure are described below in the context of an LTE cellular network, the methods and systems of the present disclosure are not so limited. Those skilled in the art will recognize that the methods and systems of the present disclosure may be used with other cellular networks during cell search operations.

2. SSS sequence detection using interference cancellation

Referring now to fig. 3, a flow diagram 300 of a method for detecting a weak power SSS sequence (e.g., NID1 value) in an LTE cellular network is described in accordance with an embodiment of the present disclosure. The method of flowchart 300 may be implemented by a processor, such as a digital signal processor, in an LTE terminal (e.g., mobile phone, laptop, pager, personal digital assistant, tablet, e-reader, etc.).

The method of flowchart 300 begins at step 302 where an initial cell search is performed by an LTE terminal to detect a high power SSS sequence in a sampled signal x (k) received over an LTE network. The received signal x (k) corresponds to a "composite" OFDM symbol composed of overlapping OFDM symbols broadcast from base stations in an LTE cellular network, and k represents a sampling instance (sampling instance). The strong power SSS sequence detected in the received signal x (k) corresponds to one of the overlapping OFDM symbols. Methods of detecting strong power SSS sequences in the received signal x (k) are known and not explained in detail herein.

In step 304, the channel on which the strong SSS sequence detected in step 302 was received by the LTE terminal is estimated. In one embodiment, the channel is estimated by performing a Fast Fourier Transform (FFT) on the received signal x (k) to transform the received signal x (k) into the frequency domain. Assuming that the OFDM symbol used in the LTE network has, for example, 128 associated time domain samples and that the received signal x (k) is sampled at a suitable rate such that it includes 128 time domain samples corresponding to the time domain samples of the "composite" OFDM symbol described above, a 128-point FFT of the received signal x (k) may be performed to recover the data carried by the 64 orthogonal tones (tones) of the "composite" OFDM symbol. The FFT of the received signal x (k) is represented as:

in equation (1), N_fft=128。

After performing the FFT, the tone of interest in the frequency domain representation of the received signal X can be extracted. The extracted interesting sound is expressed as:

wherein StartTone and EndToone are variables. In one embodiment, the variables StartTone and EndTone are determined such that the total of the tones of interest includes 63: the 31 closest tones on either side of the DC tone and the DC tone itself. In other embodiments, more or fewer tones may be extracted from the frequency domain representation of the received signal including all tones.

Then, byThe represented extracted tones of interest on the SSS may correspond to known strong power SSS sequences of the estimated channelThe SSS sequence is multiplied by C. The dot product of these two values is expressed as:

where denotes the operator of dot multiplication.

After the vector Y is obtained, the value of the vector Y at its DC pitch index (index) may be replaced with the value of one of the two pitches near the DC pitch in the vector Y, the average of the two pitches near the DC pitch in the vector Y, or some other value. Vector Y may be extended to length 64 by, for example, appending the value of the note at index 63 to the end of vector Y or by pre-appending the value of the note at index 1 to the beginning of vector Y. This vector may be transformed by determining a 64-point or 128-point Inverse Fast Fourier Transform (IFFT). To perform a 128-point IFFT, an equal number of zeros may be appended to vector Y at the beginning and end of the vector so that it changes from a 64-length vector to a 128-length vector. The IFFT of this vector is represented as:

in equation (4), N_ifft=64 or 128.

Next, by using the vector y having the highest energyOf the vector y associated with the value in (1) is an exponent Loc in the vector y_MaxAnd by using estimates of the delay spread ChanSpread of the estimated channel, the noise values in vector y can be zeroed out. Vector y is specifically zeroed as follows:

y(1:Loc_Max-2)=0; (5)

y((Loc_Max+ChanSpread+1):N_ifft)=0 (6)

wherein N is_ifftIs the value used in equation (4). The exponent Loc may be estimated, for example, by squaring each element in the vector y of equation (4) and identifying the exponent of the squared element having the maximum value_Max. Alternatively, the index Loc may be estimated, for example, after calculating a plurality of vectors in a similar manner to the vector y based on signals received earlier or later on the LTE cellular network and/or based on the same signals x (k) received by other antennas, squaring individual elements of these calculated vectors and adding or averaging corresponding squared elements of the individual calculated vectors, and finally using the added or averaged squared elements to identify the index of the added or averaged squared element having the maximum value_Max. It should also be noted that the value 2 used in equation (5) above may be arranged to be configurable and can be set to any suitable value. The method of selecting the noise value position to be zeroed out may be done in other alternative ways, e.g. selecting all values that are smaller than a threshold obtained as a scaled version of the noise power estimated for y.

After the noise value in vector y has been zeroed out, a 64-point or 128-point (as selected in equation (4)) FFT of vector y may be detected to estimate the channel on which the strong power SSS sequence was received by the LTE terminal. The FFT of vector y with its noise value zeroed out is represented as:

in equation (7), N_fft=64 or 128, depending on the N selected in equation (4)_ifft。

At step 306, an estimate of the contribution of the strong power SSS and PSS sequences to the received signal x (k) is determined using the estimated channel H obtained in equation (7). The channel estimate required at the tone location where the synchronization signal is transmitted is selected from the channel estimates H in equation (7) and is referred to as H_SSS. By estimating the channel H_SSSRespectively with known SSS sequences C corresponding to high-power SSS sequences_SSSOr to a high power PSS sequence C_PSSThe point multiplication determines in particular an estimate of the contribution of the strong power SSS and PSS sequences to the received signal x (k). The dot product of these two values is expressed as:

R_SSS=H_SSS*C_SSS (8)

R_PSS=H_SSS*C_PSS (8a)

where denotes the operator of the point multiplication.

In step 308, the estimated contribution R of the high power SSS sequence_SSSAnd estimated contribution R of strong power PSS_PSSThe tones of interest in the frequency domain representation of the received signal X can be separately derived from the frequency domain representation of the received signal X as followsAndis eliminated.

Finally, at step 310, the received signal is canceled using the interference represented in equation (9) aboveTo perform cell search for a weak power SSS sequence. Cell search for weak power SSS sequences may be performed using known techniques, which may use PPS sequences to cancel interference from the received signal as represented in equation (9 a) above.

Once detected, the low power SSS sequence may be used to obtain synchronization with the cell from which the low power SSS sequence originates and detect the physical layer identity of the cell. Obtaining synchronization with this cell and detecting the physical layer identity of this cell may be beneficial for several reasons, including handing over communications from the current cell to the cell corresponding to the weak power SSS sequence due to, for example, movement of the LTE terminal.

It should be noted that step 302-308 may be repeated again for other strong power SSS and PSS sequences detected in the received signal x (k) before proceeding to step 310. Additionally, it should also be noted that step 304 and 308 may be repeated again to remove the weak power SSS and PSS sequences detected in step 310, as opposed to the strong power SSS and PSS sequences detected in step 302.

Fig. 4 shows a block diagram of an exemplary LTE terminal 400 according to an embodiment of the disclosure. The LTE terminal 400 may be, for example, a mobile phone, laptop, pager, personal digital assistant, tablet computer, e-reader, etc. and may be used to implement a method of detecting a weak power SSS sequence (e.g., NID1 value) in an LTE cellular network using interference cancellation as shown in the flowchart 300 of fig. 3.

As shown in fig. 4, the LTE terminal 400 includes an antenna 402, a transceiver 404, and a cell search module 406. The cell search module 406 specifically includes a channel estimator 408, an interference canceller 410, and a detector 412. These modules may be implemented in hardware, software, or any combination thereof. For example, one or more of these modules may be implemented by software stored in a computer readable medium and executed by a processor, such as a digital signal processor. In other examples, one or more of the modules may be implemented by specialized hardware blocks or dedicated processors that are specifically configured to implement the functionality of one or more modules.

In operation of the LTE terminal 400, the antenna 402 is configured to receive signals transmitted over the LTE network by base stations located in different cells. These signals may be formatted according to the basic LTE frame structure shown in fig. 2 and include PSS and SSS sequences as mentioned above. The transceiver 404 is configured to down-convert and sample the signal received by the antenna 402. The down-converted and sampled portions of this signal correspond to the received signal x (k) mentioned above in fig. 3 and can be provided by the transceiver 404 to the cell search module 406 for a method of detecting weak power SSS sequences (e.g., NID1 values) using interference cancellation as shown in the flow diagram 300 of fig. 3. Specifically, detector 412 may be configured to perform steps 302 and 310 in flowchart 300, channel estimator 408 may be configured to perform step 304, and interference canceller 410 may be configured to perform steps 306 and 308. It should be noted, however, that channel estimator 408, interference canceller 410, and detector 412 may be configured to perform other functions.

3. Conclusion

The present disclosure is described with the aid of functional blocks illustrating the implementation of specific functions and relationships thereof. The boundaries of these functional blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries may be defined so long as the specific functions and relationships thereof of the functional blocks are appropriately performed.

Claims (10)

1. A method for detecting a first Secondary Synchronization Signal (SSS) sequence and a second secondary synchronization signal sequence in a received signal, the method comprising:

detecting the first secondary synchronization signal sequence in the received signal;

estimating a channel on which the first secondary synchronization signal sequence is received;

estimating a contribution of the first secondary synchronization signal sequence to the received signal using a channel estimate;

cancelling a contribution estimate of the first secondary synchronization signal sequence from the received signal to produce an interference cancelled received signal; and

detecting the second secondary synchronization signal sequence using the interference-canceled received signal.

2. The method of claim 1, further comprising:

canceling a contribution estimate of a third secondary synchronization signal sequence from the received signal prior to detecting the second secondary synchronization signal sequence.

3. The method of claim 1, wherein estimating the channel further comprises:

using the received signal and a known secondary synchronization signal sequence corresponding to the first secondary synchronization signal sequence.

4. The method of claim 1, wherein estimating the channel further comprises:

multiplying the frequency domain representation of the samples of the received signal by a known secondary synchronization signal sequence corresponding to the first secondary synchronization signal sequence.

5. The method of claim 4, further comprising:

obtaining samples of the received signal based on a position of the first secondary synchronization signal sequence in the received signal.

6. The method of claim 4, further comprising:

removing noise in the frequency domain representation of the samples based on the estimated energy of the elements in the frequency domain representation and the estimated delay spread of the channel.

7. The method of claim 1, further comprising:

acquiring synchronization with a cell using the second secondary synchronization signal sequence.

8. The method of claim 1, further comprising:

determining a physical layer cell identity of a cell using the second secondary synchronization signal sequence.

9. A terminal for detecting a first Secondary Synchronization Signal (SSS) and a second secondary synchronization signal sequence in a received signal, the terminal comprising:

a channel estimator configured to estimate a channel on which the first secondary synchronization signal sequence is received;

an interference canceller configured to generate an interference-cancelled received signal by:

estimating a contribution of the first secondary synchronization signal sequence to the received signal using a channel estimate and cancelling a contribution estimate of the first secondary synchronization signal sequence from the received signal; and

estimating a contribution of a first primary synchronization signal sequence to the received signal using the channel estimate and cancelling a contribution estimate of the first primary synchronization signal sequence from the received signal; and

a detector configured to detect the second secondary synchronization signal sequence using the interference-cancelled received signal.

10. A terminal for detecting a first sequence and a second sequence in a received signal, the terminal comprising:

a channel estimator configured to estimate a channel on which the first sequence is received;

an interference canceller configured to estimate a contribution of the first sequence to the received signal using the channel estimate and configured to cancel the contribution estimate from the received signal to produce an interference cancelled received signal; and

a detector configured to detect the second sequence using the interference-cancelled received signal.